The present invention relates to an adhesive composition.
Conventionally, to adhere metals such as aluminum, the metal surface to be adhered was generally pretreated with chemical treatment such as FPL etching and phosphoric acid anodization. It was intended to obtain a firm adhesive performance by removing the surface soiling, oil, or weak oxide film to form a strong oxide film, or toughening the surface by etching or the like.
In the case of an adhesive which requires pretreatment of the adherend chemically or physically, considering the working efficiency and total cost of adhesion, it is extremely complicated in operation for use in an assembly process of constituent parts of electric products (devices), furniture, automobiles, or the like. It is therefore considered a very important improvement to change the design of the adhesive so as to exhibit a favorable adhesion without resort to the existing pretreatment process.
There have been proposed adhesives presenting a favorable adhesion by applying them directly on the metal surface without carrying out pretreatment. For example, the Japanese Official Patent Provisional Publication, Showa 63-6074, has proposes a water-based dispersion adhesive prepared by adding monovalent sodium phosphate or trimethyl phosphate to an ethylene-based ionomer resin dispersion. This adhesive is used in adhesion of aluminum foil with the ethylene-based ionomer resin film or polyester film, and as compared with other adhesives not containing sodium phosphate or trimethyl phosphate, the 180.degree. peeling-off strength is improved. The Japanese Official Patent Gazette, Heisei 2-8633, has proposes a curable polyurethane adhesive composition comprising diphenylmethane diisocyanate modified matter, polyol, and organic phosphate compound possessing at least one --SH group. This adhesive is used in adhesion of iron piece treated by sand blasting, and stainless steel, aluminum, copper, zinc, tin, and zinc plated chromate-treated material, and the peeling strength is improved as compared with an adhesive not containing the organic phosphate compound.
The adhesives proposed in these two publications possess a satisfactory adhesive power, but the adhesive strength is not sufficient when used in applications requiring greater adhesion property such as adhesion of metal to metal for automobile.